The present invention relates generally to medical imaging systems and, more particularly, to receive sensitivity correction in magnetic resonance imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (i.e., an excitation field referred to in the art as B1+ and as opposed to a receive magnetic field known in the art as B1−) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1+ is terminated and this signal may be received as a B1− signal and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
There are a variety of known techniques to determine if the B1+ field produced by a magnetic resonance coil or array is homogeneous or to what degree the field is inhomogeneous. Such techniques are often referred to as B1+ mapping (and is commonly referred to in the art as “B1 mapping” without the “+” designator). In general, B1+ mapping techniques may either implement spatially or non-spatially resolved B1+ measurements. B1+ measurements are spatially resolved if one or more spatial encoding gradients are applied during acquisition and, in contrast, B1+ measurements are non-spatially resolved when spatial encoding gradients are not utilized during B1+ measurements. Among other things, B1+ maps can be used to adjust transmit gain to produce a radio frequency (RF) pulse at a specific flip angle or to design multi-transmit channel RF pulses, as examples. B1+ mapping can also serve as an aide in T1 mapping and/or other quantitative MR imaging techniques. Some B1+ mapping techniques are T1 dependent. That is, the signal utilized for B1+ is often weighted as a function of T1 relaxation. Other B1+ mapping techniques are B0 or chemical shift dependent. Still other techniques are inaccurate over certain ranges of B1+ field, and/or are dependent on large RF power depositions.
Thus, B1+ mapping is employed in known techniques to account for and correct for transmit B1+ field nonuniformity in MR imaging. However, in high field MR, such as 3T or higher, images are affected by both a transmit sensitivity (non-uniform B1+) as well as a receive sensitivity (B1−) at the same time. Both transmit and receive non-uniformity contribute to image artifacts that are not distinguishable or separable using known techniques in high field MR. As such, receive sensitivity correction methods often ignore the presence of a non-uniform B1+ field and assume a homogenous B1+ field. One known method estimates the receive sensitivity (B1−) using the transmit sensitivity (B1+), which typically only are used with transmit/receive (Tx/Rx) coils and not with receive only phase array coils.
It would therefore be desirable to have a system and apparatus that efficiently distinguishes and accounts for B1+ and B1− fields of a magnetic resonance system.